The present invention relates to air/fuel ratio sensors used for internal combustion engines or other combustion engines, and more specifically to air/fuel ratio sensors of a type having an oxygen sensor cell and an oxygen pump cell.
Japanese patent provisional publication No. 60-236056 discloses one conventional example of such air/fuel ratio sensors. As shown in FIG. 7, the oxygen sensor cell of this example has a primary element of metal oxide, such as a transition metal oxide, whose electrical resistance changes with change in oxygen concentration. The oxygen pump cell has a member of oxygen ion conductive solid electrolyte.
Japanese patent provisonal publication No. 61-30758 discloses another conventional example which is shown in FIG. 20A. The oxygen sensor cell of this example is composed of a substrate of oxygen ion conductive solid electrolyte, which is sandwiched between a reference electrode in direct contact with a reference gas such as air, and a measuring electrode in direct contact with a gas to be measured.
However, these air/fuel ratio sensors are unsatisfactory in that the burden of the oxygen pump cell is too much, and the tendency to overshoot in a transient response is too high, as explained hereinafter more in detail.